Television and like system



p 1942- w. JONES ET AL 2,295,346

TELEVISION. AND LIKE SYSTEM Filed Aprfil 21, 1939 I 12' 12' 13 10 11 F:g. 4. i l Inventors ;3 ffavw/sws Patented Sept. 8, 1942 UNI-TED. STATES PATENT OFFICE TELEVISION AND LIKE 'SYSTEM William Jones and BadenJohn Edwards, Cam- 7 bridge, England, assignors to Pye Limited,Cambridge, England, a British company Application April 21, 1939, SerialNo. 269,229 In Great Britain April 22, 1938 I 4 Claims.

The present invention relates to electrical picture transmission systemssuch as television,

phototelegraphy and the like, and more particularly to a circuitarrangement for the separation of the frame synchronising impulses fromthe line synchronising impulses and the picture intelligence.

In the transmission of television two sets of synchronising impulses areusually transmitted;

synchronising impulses and the frame synchronising impulses aregenerally of substantially the same amplitude but are of differentduration, the frame impulses being oflonger duration than the lineimpulses.

Difficulties exist in separating the frame from the line synchronisingimpulses in the receiver, particularly when interlaced scanning of thepicture is employed. The present invention'has for its object to providean improved method of effecting such separation and also for separatingthe frame synchronising impulses from the picture intelligence.

A further object of the invention is to provide a circuit arrangementfor separating the frame impulses from the picture intelligence and lineimpulses in a single electronic discharge device,

and without the necessity of firstseparating the synchronising impulsesfrom the picture intelligence.

The invention will now be described by way of example with reference tothe accompanying drawing in which Fig. 1 shows one embodiment of acircuit arrangement according to the invention.

Figs. 2 and 3 show one type of wave form and the corresponding operationofthe circuit.

Figs. 4 and 5 show an alternative type of wave form and thecorresponding operation of the circuit.

Referring to Fig. l which shows a circuit arrangement for efiecting theseparation of frame impulses from the line impulses and pictureintelligence, the arrangement comprises a pentode or other valve VI, tothe input terminals l of which is applied the received video signals(rectified transmitted wave-form). The input circuit includes a seriesresistance RI and a condenser C! in shunt between the grid and cathodeof the valve, or, as shown, between the grid and earth. The valve anodeload is constituted by a resistance R2 and the coupling to the time baseunit T is effected through the condenser C2. Any other alternative formof connection between the valve and the time base unit, such as atransformer, may, however, be'employed. A suitable positive potential isimpressed upon the screening grid of the valve from a potential dividerR3, R4, R5 connected across the anode current supplylead, the negativeterminal of which is earthed. A further tapping point 2 is provided uponthe resistance R5 whereby a suitable bias may be applied between thecontrol rid and the cathode. C3 and C4 are the usual by-pass condensersto the cathode and screening grid respectively.

The valve employed preferably has a steep slope so that a small changein the potential applied to the control grid can shift the operatingpoint from anode current cut-off to anode current saturation.

The method of operation of the circuit will now .be described.

Assuming in the first case that the received video signal is in the formshown in Fig. 2 and comprises picture intelligence ll, linesynchronising impulses l2 and frame synchronising impulses l3. The lineI0 represents a datum line corresponding to black in the picture signalsso that the synchronising impulses are transmitted in theblacker-than-black sense. Both the line and frame impulses are fsubstantially the same amplitude but the frame impulses are of longerduration than the line impulses.

is reduced is different in each case. As shown in Fig. 3 the valve is sobiassed that the potential applied to the grid during pictureintelligence and line impulses still lies within the region of anodecurrent saturation whereas during frame impulses the grid potential isreduced so that the operating point is transferred through the slopingpart of the characteristic to'or beyond the point of anode currentcut-off whereby noise usually apparent on the base of the synchonisingpulse is eliminated and improved synchronisin is obtained. If the timeconstant of the charge circuit Cl, BI is substantially of the sameduration as the duration of the frame impulses, and the valve is biassedto the point of anode current saturation the operating point onthe valvecharacteristic will be shifted to the point of anode current cut-offduring the reception of frame impulses. The time constant of the chargecircuit RI, Cl, however, is preferably longer than the duration of theframe impulses, but in this case in orderthat the frame impulses mayshift the operating point to the point of anode current cut-01f thesteady'biassing potential applied to the control grid will have to beadjusted by varying the position of the tapping 2. The time constant ofthe charge circuit could be less than the duration of the frame impulsesprovided that it is greater than the duration of the line pulses, but inthis case the adjustment would be more critical.

If the video signal is in the form shown in Fig. 4 in which thesynchronising impulses are transmitted in the whiter-than-white sense,the valve. should be biassed positively, such as to the point of anodecurrent saturation and the picture intelligence and line synchronisingimpulses would, in this case, lie in the region of anode current cut-offas shown in Fig. 5. In this case the condenser Cl is charged up by thesynchronising impulses instead of being discharged as in the previousexample.

In both cases it is only the frame synchronising impulses which areamplified, and the amplified frame impulses are then fed to the frametime base T.

The system of separation according to this invention has severaladvantages including the complete separation of the line and frameimpulses before amplifying. It is usual to have a four to onediscrimination between the two sets of impulses and an amplitude filteris arranged to pass only the frame impulses. In the arrangementaccording to the invention, however, the discrimination is infinite asno line impulses are amplified. Further, the equipment necessary forseparating the frame impulses from both the line impulses and pictureintelligence comprises only one valve which acts as a synchronisingpulse separator and amplifier.

It will be understood that by varying the working point on the valvecharacteristic the circuit arrangement may be used for the separation offrame impulses in other types of video signals, or video signals appliedin a different phase.

We claim: 1

1. Circuit arrangement for separating and simultaneouslyv amplifyingframe synchronizing impulses from a composite signal including frame andline synchronizing impulses of substantially the same amplitude but ofdifferent duration,

comprising a single amplifying device containing cathode, control gridand anode electrodes therein and having a transmission characteristiccurve with a steep sloping portion terminating at each end in asubstantially horizontal portion, an input circuit having apredetermined time constant comprising a resistance and capacitance soas to be responsive to the duration of a signal apphed to said inputcircuit for varying the biasing 120+ tential applied to said amplifyingdevice and thus the operating point on said curve, means for applyingthe frame and line synchronizing impulses to said input circuit,

point of said amplifying device so lying on one substantially horizontalportion of said curve that the biasing potentials applied to theamplifying device due to linesynchronising impulses cause the amplifyingdevice to continue to operate on said substantially horizontal portionto produce negligible output change, while the biassing potentialsapplied to the amplifying device due to frame synchronising impulsescause the operating point on the curve to swing over to the othersubstantially horizontal portion of said curve through the steep slopingportion of said characteristic curve, and an output circuit from saiddevice connected to said plate electrode in which said framesynchronising impulses appear as a square topped waveform.

2. Circuit arrangement as claimed in claim 1, wherein the compositesignals applied to said in ut circuit comprise also pictureintelligence, the initial operating point of the amplifying device solying on the one horizontal portion of said curve that the biassingpotentials applied to the amplifying device due to picture intelligencealso cause the operating point of said amplifying device to remain onsaid substantially horizontal portion to produce negligible outputchange.

3. Electronic valve circuit arrangement employing a single electronicvacuum valve for sep-' arating frame synchronising impulses from acomposite signal including frame and line synchronising impulses ofsubstantially the same amplitude but of different duration and pictureintelligence, comprising a single electronic vacuum valve having acathode, a control grid and an anode, and being of the type having atransmission characteristic curve with a steep sloping portionterminating at each end in a substantially horizontal portion, areactive impedance having a predetermined time constant and comprisingresistance and capacitance, means for applying the frame and linesynchronizing impulses and picture intelligence to the control electrodeof the valve through said reactive impedance, and means for biassing'thecontrol electrode to such a point on the transmission characteristic ofthe valve that the potentials applied to the control grid from thereactive impedance due to line synchronising impulses and pictureintelligence cause the valve to operate on one of the substantiallyhorizontal portions of the transmission characteristic of the valve,while the potentials applied to the control electrode from the impedancedue to the frame synchronising impulses cause the ent duration,comprising an electronic amplifying device havinga cathode, a controlgrid and an anode, and having a control grid voltage/anode currentcharacteristic curvehaving a steep sloping portion terminating at eachend in a substantially horizontal portion, means for applying a biassingpotential to said control grid so as to bring the operating point ofsaid amplifying device initially on one of the substantially horizontalportions of the characteristic curve, a rethe initial operating activeimpedance having a predetermined time constant, said reactive impedancecomprising a zontal portions of the characteristic curve to pro-- ducenegligible output change, while the varia-- tions of the biassingpotential due to frame synchronising impulses cause the operating pointon v the characteristic curve to swing over to the other substantiallyhorizontal portion of said characteristic curve through the steepsloping portion of said characteristic curve, whereby in said outputcircuit only the amplified frame synchronisingim- 10 pulses appear as asquare topped wave.

BADEN J. EDWARDS. WILLIAM JONES.

